1. Field of the Invention
The present invention relates to a large screen display device, more particularly to a light source display tube to constitute a picture element of a color display device.
2. Description of the Prior Art
When a monochromatic display tube utilizing light emission of fluorescent material forms a single picture element and such monochromatic display tubes are arranged in matrix form to constitute a large screen display device, a disadvantage occurs in that a space is produced on the connecting portion of each monochromatic display tube, thereby, improvement of resolving power is difficult, and high resolving power may be accompanied by high cost.
Consequently, in the prior art, a light source display tube surmounting the above-mentioned disadvantage is disclosed in a light source display tube which was invented by Kobayashi et al. and filed in the Japanese Patent Office by Ise Denshi Kogyo Co., Ltd. on July 8 , 1985 and laid open on Jan. 19, 1987as Japanese Patent Laid-Open Publication No. 62-10849.
The light source display tube in the prior art is shown in a front view of FIG. 1, a lateral sectional view of FIG. 2 and an exploded perspective view of FIG. 3. The figures show the case wherein a fluorescent screen having respective fluorescent materials of R (red), G (green) and B (blue) each forming one picture element is arranged in matrix form having 3.times.3 picture elements in number.
In these figures, numeral 1 designates as a vacuum envelope a glass tube which is hermetically sealed by a front panel 2, a back plate 3 and a rectangular side plate 4. On the inner surface of the front panel 2 is formed a fluorescent display member 5 which is arranged and coated in matrix form using three-color fluorescent materials R, G, B each forming a unit picture element, and comprises fluorescent screens 5R, 5G and 5B of the 3.times.3 picture elements, wherein suffixes of the fluorescent screens 5R, 5G and 5B correspond to red (R), green (G) and blue (B) respectively.
Numeral 6 designates an anode electrode group comprising a plurality of accelerating anodes 6.sub.1, 6.sub.2, . . . arranged corresponding to a periphery of the fluorescent screens 5R, 5G and 5B of the fluorescent display member 5 respectively, and a high voltage is applied to these accelerating anodes 6.sub.1, 6.sub.2, . . . through an external terminal 16.
Numeral 7 designates a cathode electrode group comprising cathodes 7.sub.11 -7.sub.33 for electron emission (7.sub.13, 7.sub.23, 7.sub.31, 7.sub.32 and 7.sub.33 not being shown) respectively arranged independently corresponding to each of the fluorescent screens 5R, 5G, 5B of the fluorescent display member 5, and these cathodes 7.sub.11 -7.sub.33 are supported between a pair of supports 17a, 17b, wherein first and second suffixes of the cathodes 7.sub.11 -7.sub.33 correspond to the first-third rows and the first-third columns respectively.
In each of the cathodes 7.sub.11 -7.sub.33, for example, an indirectly-heated cathode where oxide is coated on a nickel sleeve or a direct-heated cathode where oxide is coated on a tungsten wire may be used.
Numeral 8 designates a grid electrode group comprising control grids 8.sub.1 -8.sub.3 for row selection arranged between the cathode electrode group 7 and the fluorescent display member 5, and these control grids 8.sub.1 -8.sub.3 are provided with holes 9.sub.1 -9.sub.3 for electron passage so that an electron beam 11 from the cathodes 7.sub.11 -7.sub.33 respectively passes as a divergent beam in the direction corresponding to each of the fluorescent screens 5R, 5G and 5B of the fluorescent display member 5.
Numeral 10 designates a back electrode group comprising stripe-form back electrodes 10.sub.1 -10.sub.3 for column selection, respectively arranged to face each of the fluorescent screens 5R, 5G and 5B of the fluorescent display member 5, along the column direction, behind the cathode electrode group 7, on the back plate 3 of the vacuum envelope 1, and these back electrodes 10.sub.1 -10.sub.3 are a conductive layer of silver or the like formed on the back plate.
Each of the back electrodes 10.sub.1 -10.sub.3 is given either negative potential and 0 V or positive potential of several V with respect to the potential of each of the cathodes 7.sub.11 -7.sub.33 so as to control the electron beam 11 emitted from the cathodes 7.sub.11 -7.sub.33.
Numeral 12 designates a lead wire as an external terminal to extend each electrode of the cathode electrode group 7, the grid electrode group 8 and the back electrode group 10 from the back plate 3 to the outside.
Next, operation will be described. When each of the back electrodes 10.sub.1 -10.sub.3 is at negative potential with respect to the potential of the cathodes 7.sub.11 -7.sub.33 since the circumference of these cathodes 7.sub.11 -7.sub.33 is surrounded by the negative potential, electrons from each of the cathodes 7.sub.11 -7.sub.33 cannot flow through the control grids 8.sub.1 -8.sub.3 to the accelerating anodes 6.sub.1 -6.sub.2, . . . ; thereby, the cutoff state occurs.
In this state, if 0 V or positive potential of several V with respect to the potential of the cathodes 7.sub.11 -7.sub.33 is applied to the back electrodes 10.sub.1 -10.sub.3, the electron beam 11 emitted from these cathodes 7.sub.11 -7.sub.33 flows towards the control grids 8.sub.1 -8.sub.3.
If the potential of each of the control grids 8.sub.1 -8.sub.3 is negative potential with respect to the cathodes 7.sub.11 -7.sub.33 then, the electron beam 11 cannot pass through the electron passing holes 9.sub.1 -9.sub.3 of these control grids 8.sub.1 -8.sub.3, thereby the electron beam 11 can not flow to the accelerating anodes 6.sub.1 -6.sub.2, . . . , and each of the fluorescent screens 5R, 5G and 5B of the fluorescent display member 5 does not emit light.
If the potential of the control grids 8.sub.1 -8.sub.3 is a positive potential with respect to the cathodes 7.sub.11 -7.sub.33, the electron beam 11 passes through the electron passing holes 9.sub.1 -9.sub.3 of the control grids 8.sub.1 -8.sub.3 respectively, thereby each of the fluorescent screens 5R, 5G and 5B emits light.
Consequently, each of the grid electrodes 8.sub.1 -8.sub.3 of the grid electrode group 8 arranged in matrix form corresponding to each of the fluorescent screens 5R, 5G and 5B and each of the back electrodes 10.sub.1 -10.sub.3 of the back electrode group 10 are selectively controlled to provide drive (dynamic drive); thereby, only the fluorescent screens 5R, 5G and 5B with both electrodes crossing can emit light selectively.
As above described, the fluorescent screens 5R, 5G and 5B composed of the fluorescent materials of the three colors are arranged in matrix form of 3.times.3 picture elements on the inner surface of the front panel 2 of the vacuum envelope 1, and the cathode electrode group 7, the grid electrode group 8 and the back electrode group 10 are installed corresponding to each of the fluorescent screens 5R, 5G and 5B, thereby a light source display tube of high brightness emission can be obtained.
Consequently, when a large screen color display device is assembled using the light source display tube as a unit, a space between each picture element is shortened in comparison to that using a monochromatic tube having only one picture element, thereby the resolving power can be improved and the number of parts and the number of manufacturing processes can be decreased. Not only can the structure be simplified and the cost be made low, but also the weight of the display device can be reduced.
In the example shown, although the fluorescent screens composed of the fluorescent materials of the three colors, R, G and B are arranged in matrix form of 3.times.3 picture elements on the inner surface of the front panel 2, the example is not limited to this but a fluorescent screen with one fluorescent material forming one picture element may be arranged in matrix form of m.times.n picture elements (where m, n are arbitrary positive integers) within the vacuum envelope, and corresponding to this, the arrangement and constitution of the grid electrode group and the back electrode group can be varied.
Also as shown in FIG. 4, a light source display tube is proposed wherein each control grid of the grid electrode group 8 is made of channel form with a U-shaped cross-section, and a shield plate 18 made of metal projecting laterally is attached to each such control grid, thereby stray electrons from the gap between each control grid and the back plate are absorbed by the shield plate 18 and pseudo emission of the fluorescent screen due to the stray electrons is effectively preveated (for example, see Japanese utility model application No. 62-114562).
Further, as disclosed in Japanese patent application laid-open No. 62-241256 for example, an electron passing portion formed on each of the control grids 8.sub.1 -8.sub.3 as described above is provided with a dome-shaped mesh portion, thereby electrons emitted from the cathodes 7.sub.11 -7.sub.33 are spread uniformly and can be irradiated uniformly onto the fluorescent screen 5, and the divergent angle of the electron beam 11 can be arbitrarily adjusted corresponding to the curvature of the dome-shaped mesh portion.
Moreover, a light source display tube is proposed where the larger the curvature of the dome-shaped mesh, the larger the divergent angle of the beam, thereby the length of the display tube can be decreased.
As shown in FIGS. 4 and 6, when the channel-shaped control grids 8.sub.1, 8.sub.2, . . . with a U-shaped cross-section are arranged on the back plate 3, in order to extend the lead wires 12 for the cathodes 7.sub.11, 7.sub.12, . . . , the back electrode 10.sub.1 or the like, notched recesses 13 must be formed on both lateral edge portions of the control grid abutting on the back plate 3.
On the other hand, electrons emitted from the cathodes 7.sub.11, 7.sub.12, . . . are attracted by a voltage applied to the control grids 8.sub.1, 8.sub.2, . . . and apt to move to both lateral sides of the control grid. Consequently, stray electrons occur from the notched recess 13, thereby pseudo emission may be produced.
Also if a control grid manufactured by means of etching and press forming is used as the channel-shaped control grid with a U-shaped cross-section, when the electric field of 10.sup.8 V/m or more is applied to the control grid, electron emission is produced due to the field emission phenomenon and pseudo emission may be produced at the fluorescent display member due to the emitted electrons.
In this case, if the control grid is manufactured by drawing and the surface is rounded, the field emission can be prevented, but the working of the dome-shaped mesh portion becomes difficult.
Also as shown in FIG. 6, if a getter 16 is attached to a skirt portion of the accelerating anode 6.sub.1, a getter film 17 is formed in a wide area of the inner surface of the side plate due to flash of the getter 16 thereby electrical trouble such as shortcircuit or discharge may be produced between the getter film 17 and the control grid 8.sub.1.
Since the cathodes 7.sub.11 -7.sub.33, the control grids 8.sub.1 -8.sub.3, the back electrodes 10.sub.1 -10.sub.3 and lead wires for these electrodes are installed on the back plate 3 of the vacuum envelope 1, it is difficult to widen the space between each electrode and the space between each lead wire, thereby electrical trouble may be produced also on account of this condition.
In order for the lead wires 12 for the cathodes 7.sub.11 -7.sub.33, the control grids 8.sub.1 -8.sub.3, the back electrodes 10.sub.1 -10.sub.3 and the like to be easily taken to the outside, i.e., to the rear side of the back plate 3, as shown in FIG. 7, the vacuum envelope 1 is manufactured in trumpet shape so that the installation area of the back electrodes 10.sub.1 -10.sub.3 becomes narrower than that of the fluorescent screen.
In this case, the back electrode 10.sub.1 and the control grid 8.sub.1 being next to or near the side plate of the vacuum envelope 1, must be installed not just behind the corresponding fluorescent screen 5R but in a position partially facing the next fluorescent screen 5B, i.e., position-shifted towards the center of the vacuum envelope 1. Consequently, electrons emitted from the cathode 7.sub.12 are guided towards the fluorescent screen 5R and also may be leaked to the next fluorescent screen 5B, thereby pseudo emission may be produced in the fluorescent screen 5B.
Also when electrons emitted from the cathodes 7.sub.11 -7.sub.33 respectively are accelerated by the accelerating anodes 6.sub.1, 6.sub.2, . . . , a part thereof charges a top end edge portion of the skirt of the accelerating anode extending towards the back plate, and discharge is produced at the edge portion. Consequently, the divergent beam may not be irradiated uniformly and efficiently from the cathodes 7.sub.11 -7.sub.33 onto the fluorescent screens 5R, 5G, 5B respectively.